Make comments copyable via adding an (optional) isCopyable method to ICopyable

[cpcallen]

There are IMHO a number of significant issues with #9107, including:

• Making it possible to copy blocks even if their .isDeletable() or .isMovable() methods return false, in contradiction with our previous decision not to do that in v12 (see Cannot copy a shadow block #9084).
• Creating an isCuttable helper function that is not guaranteed to be in sync with isCopyable (thus contradicting the purpose of having factored isCopyable out of the cut and copy preconditionFns in in the first place), and
• Making it impossible to cut or copy workspace comments.

I think #9107 should be reverted (except for new tests), and a different approach taken to resolving RaspberryPiFoundation/blockly-keyboard-experimentation#546—ideally by ascertaining what change caused flyout blocks to have become uncopyable in the the first place and narrowly addressing that.

Further increasing the range of things that can be copied is highly desirable but should be left until we decide we want to implement the proposed changes described in #9098 (or some comparable alternative approach).

Originally posted by @cpcallen in #9107 (review)

[RoboErikG]

The first two changes are intentional, though not allowing comments to be copied was not.

• isDeletable() and isMovable() check if the workspace the block is on is editable which is intended behavior (since you can't delete or move blocks on a read-only workspace). In the case of copying, we want to allow copying even if the workspace is read only because a copy doesn't modify the workspace it's on. This is what isOwnDeletable() and isOwnMovable() are for. This also was a simple way of allowing shadow blocks to be copied which I think from reading the linked discussions is the desired behavior but had been deprioritized.
• isCuttable() should check if the workspace is editable. Cutting a block modifies the workspace, which shouldn't be allowed if the workspace isn't editable. This is a significant and intended difference between cut and copy and requires different checks, but it would be functionally equivalent to have it call isCopyable() and then add additional checks so that's a reasonable refactor.

@cpcallen are you alright with changing the bug title to reflect that workspace comments should be copyable? The other behavior changes are intended. I also agree with your proposal in #9098 that the right long term solution is to add an isCopyable() method to ICopyable (which I think should check isOwnDeletable and isOwnMovable).

[RoboErikG]

After discussion, the planned fix is to add an optional isCopyable() method to ICopyable and make blocks and comments implement it with the current checks (and also using those as the fallback). In addition, isCuttable() should call isCopyable() and then add additional checks for the workspace being editable.